Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis

In order to reduce carbon dioxide emission, one solution is to convert into valuable chemicals or fuels, e.g. transforming CO₂ into CO by electrochemical reduction. Thus, this greenhouse gas could be re-used in particular as syngas (CO + H₂) by co-electrolysis of CO₂/H₂O. High temperature electrolysis cells can be the best energetic devices to produce such syngas. In particular, molten carbonates are known to solubilize CO₂ very significantly higher than other solvents. Therefore, it is compulsory to investigate and understand the mechanism of CO₂ reduction in such media to consider its further use and valorisation. The present study is a critical approach aiming at elucidating the mechanisms for CO₂ electroreduction, using an inert Pt electrode in the molten eutectic Li₂CO₃–K₂CO₃ (62-38 mol%), at 650 °C, under different partial pressures of CO₂. Complementary electrochemical techniques, including sweep square-wave voltammetry and relaxation chronopotentiometry, were carried out. Their combination allowed us to evidence that the electroreduction of CO₂ into CO is feasible in oxo-acidic conditions, involving a diffusion-limited quasi reversible system in a one electron-step.